This application claims priority to an application entitled xe2x80x9cMulti-Band Voltage Controlled Oscillator Circuitxe2x80x9d, filed in the Korean Industrial Property Office on Apr. 30, 2001 and assigned Serial No. 2001-23292, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO), and more particularly to a multi-band voltage controlled oscillator circuit that generates a multi-band oscillator frequency signal and has a band-switchable resonant circuit.
2. Description of the Related Art
As is well known in the art, a radio communication system employing a super heterodyne system uses a local oscillator frequency signal to up-convert a modulated baseband signal into a radio frequency (RF) signal or to down-convert a received RF signal into a baseband signal. In this regard, the radio communication system has at least one oscillator capable of generating a local oscillator frequency signal of a desired band.
FIG. 1 is a circuit diagram illustrating a basic construction of a Colpitts oscillator, which is typically used in a radio communication system.
Referring to FIG. 1, the Colpitts oscillator comprises a resonator 110, a feedback circuit 120 and an amplifier 100. The resonator 110 includes varactors CV1 and CV2 connected to ground, and an inductor L connected in parallel to the varactors CV1 and CV2. The varactors CV1 and CV2 conduct based on a control voltage to determine a resonance frequency in association with the inductor L. The feedback circuit 120 feeds an output signal (having the resonance frequency) from the resonator 110 back to the amplifier 100. The amplifier 100 amplifies the signal fed back from the feedback circuit 120 and transfers the amplified signal again to the resonator 110. As a result, the Collpitts oscillator generates a resonance frequency signal of sufficiently high amplitude and a desired band.
The resonance frequency f can be defined as in the following equation 1:                     f        =                  1                      2            ⁢            π            ⁢                                          L                *                                  (                                      CV1                    //                    CV2                                    )                                                                                        [Equation  1]            
In the oscillator illustrated in FIG. 1, operating in the above manner, the varactors CV1 and CV2 vary in capacitance with the control voltage, resulting in a variation in the resonance frequency. For this reason, the oscillator of FIG. 1 is typically referred to as a voltage controlled oscillator (referred to hereinafter as VCO). This VCO is widely used in radio communication systems requiring a frequency channel switching operation, in that it has the advantage of varying a resonance frequency with a control voltage.
With the development of radio communication techniques, a radio communication system supporting CDMA (Code Division Multiple Access) has a frequency band expanded from 800 MHz (IS-95 and IS-54) to 1900 MHz (ANSI J-STD-008 and DCS1800). Moreover, the introduction of the IMT-2000 technology development brings with it the active use of a frequency band of 2.0 GHz or more by a radio communication system. For communication with both of the systems using such different frequency bands, a specific radio communication system must support both of the frequency bands. In this regard, a radio communication system supporting radio frequencies of different bands requires a multi-band voltage control oscillator circuit capable of generating multi-band local oscillator frequencies associated with the radio frequencies.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a VCO circuit for generating multi-band local oscillator frequencies.
It is another object of the present invention to provide a VCO circuit, which comprises a band selection circuit for adjusting a resonance frequency for selective adjustment of an output frequency in a radio communication system supporting radio frequencies of different bands.
In accordance with one embodiment of the present invention, the above and other objects are accomplished by utilizing a multi-band voltage controlled oscillator circuit comprising a resonant circuit operated in response to a control voltage, the resonant circuit having a resonance frequency determined depending on the control voltage; and at least one band selection circuit connected in parallel to the resonant circuit for adjusting the resonance frequency in response to a unique band selection signal.
In accordance with another embodiment of the present invention, there is provided a multi-band voltage controlled oscillator circuit comprising a resonant circuit and at least one band selection circuit. The resonant circuit includes at least one varactor, at least one capacitor and at least one inductor operated in response to a control voltage. Also, the resonant circuit has a resonance frequency determined according to capacitances of the varactor and capacitor and an inductance of the inductor. The one band selection circuit adjusts the resonance frequency in response to a band selection signal. The band selection circuit includes at least one capacitor connected in parallel to the resonant circuit, and a switching element for turning on/off the capacitor in response to the band selection signal.